Process of producing cellulosic pulp



March 26, 1940. T. DUNBAR l PROCESS OF PRODUCING CELIIULQSIC PULP Original Filed Nov. 22, 1935 2 Sheets-Sheet 1 www March 26,v 1940.V

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Patented Mar. 26, 1940 UNITED STATES PATENT OFFICE PROCESS F PRODUCING CLLULOSIO v PULP n Thomas L. Dunbar, Watertown, N. Y., assignor to Chemipulp Process, Inc., Watertown, N. Y., a

corporation of New York Application November 22, 1935, Serial No. 51,162 Renewed August 22, 1939 20 Claims.

This invention relates to improvements inthe treatment of cellulosic materials, more particularly to an improved method of and apparatus for producing pulp.

The present invention pertains' to the production of pulp by alkaline digestion, that is to say to the soda process or a modification thereof such as the well known sulphate process.

As is known the soda process of treating wood presents some disadvantages.

'As compared to the sulphite process, the soda process results in a greater loss in cellulosic material due toy the excessive degradation of the woodyconstituents caused by the rather drastic action of the caustic on the wood. Again, although the chemical reactions involved are relatively rapid, the entire process requires considerable time because of the prolonged period involved in washing the pulp. Yet again while the recovery of soda as a chemical process is comparatively simple, yet it adds considerably tothe cost of the pulp. The apparatus involved in the recovery sive section of the milland the efliciency of the step is the most expenrecovery system is' the most important factor in operative cost. According to the pres are e'ected throughout the production process.-

ent method, economies The present improved method involves the optimum utilization of heat units of the system to preheat the digestion liquor; this digestion liquor is subsequently utilized toV impregnate and cook the cellulosic material. ture in the digester itself simple and Yeconomic circulating system. The

improvement also involve Uniformity of temperais insured by utilizing a s a special type of feeding circuit whereby the chips are contacted with the digestion liquor in su ch a manner as to provide for uniform reaction throughout the cellulosic mass in the digester.

'It is thus `an object of the lpresent invention `to provide an improved method of digesting pulp with `alkaline reagents.

Another object is to a economies in the production of pulpby the soda. (or equivalent) process.`

chieve marked thermal lAfurther object is to `provide` a .novel method y.of producing pulp by thesodaV or sulphate process,

of either kdirect or indirect permitting the .use

steaming.`

Ahstill further object isto insure economies. in the-caustic recovery System.. ,Y

Yet another lobject is todiminish concentration of the kcooking liquor wh ile producinga; V*pulp of a method involving either direct steaming or in-- direct steaming. By reason of a specialI flow .of the uids during the treatment marked economies in the subsequent recovery system are attained.

In order more clearly to describe the invention, preferred embodiments are shown in the accompanying drawings, in which:

Figure 1 is a diagrammatic illustration of one type of system involving the direct steaming.A

Fig. 2 is a view similar'to Fig. 1 showing a -modified' or indirect steaming method.

As will be seen upon reference to Figure l, the major elements in the system include the digester I and the accumulator 2; 'Ihe digester may be of the usual type employed in the art and is provided with a detachable manhole cover 3 permitting the admission of chips, and the blow-0E line 4, controlled by valve 4', permitting charge of the digested pulp.

'I'he accumulator 2 may be of .anyzsuitable size and construction and preferably should-havea the i dis-v capacity for three of four digesterrcharges. xf The accumulator may be tubular in rshape (as shown) x or may be of spherical coniigurati'on; A-feed line 5 leads from the make up system and discharge strong caustic liquor into the accumulator at the upper or any other desired section. vA uid discharge line 6 controlled by` valve 6 is con-` nected to the accumulator, preferably at the lower portion thereof, through which liquor may be forced into digester I or any otherA digester of a battery. If desired theaccumulator 2 may be elevated with respect to the digester I so as to advantageously utilize a gravityflow in filling the digester.

The-feeding circuit likewise includes the4 pump 'I,4 the discharge of lwhich is connectedto line y8. vAt its upper end line 8, is providedwith the control valve 8 and connects with header 9. From header 9 a line I0 leads into the upper portion of `the digester through a suitable packing I I.,

Interiorly of the vdigester the line I0 communicates with the axially positioned pipexIZ.l

As shown, this is positioned substantially on the longitudinal axis of the digester and is formed with the perforations I3. An expansion joint I4 is interposed in the distributor'pipe I2 so as to permit linear expansion due to thermal changes. While the single joint is shown positioned at or near the center of the pipe, it will be appreciated that such joint may be placed at one or both ends of the pipe or at any other suitable point.

'I'he lower end of the pipe I2 is connected through line I5 to the pump IB. The pipe I5 is provided With a suitable bushing or packing I1 to insurea tight joint at the point where it projects through the digester. of pump:y I6 is connected through line I8 to the closed coil I9 located within the accumulator. Suitable glands or packings 20 and 20 are provided at each end of the coil I9 to insure a pressure tight connection.

The digester likewise is provided with a top relief line 2| having a control valve 2|' which 3| which leads to the reclrculator 24. This recirculator may be of any suitable type, such as the well known eductor whereby high velocity fluids such as steam passing through line 3| establishes an. area of reduced pressure within. the element 24 and induces a flow of uid through the line 23. I'he discharge side of recirculator 24 is connected to the recirculating pipe 32 which, as shown, enters the bottom of the digester. 'Flow of fluids through this recirculating line may be controlled by the valve 32.

'Ihe digester blow-oil line 4 leads, as shown, to the blow-off tanke40. 'I'his may be of any desired type known inthe art and may comprise the typical blow-pit or any suitable form of steam separator in which the pressure is suddenly released to separate the steam and by reason of the violent evolution of the steam to effect disintegration of the chips to the fibrous condition.

From the tank or separator 40 the stock flows by gravity to the washing pans 42 and 43. These may be of the well known construction and may comprise iron tanks having perforated false bottoms. In the pans the stock is washed either by first flooding with weak liquorfrom a previous wash and subsequently with hot water which pro'- duced the weak liquor for the first wash and finally with hot waterv which latter was run. to the sewer.

As will appear more fully hereinafter, the alkali recovery system of the present invention differs from prior practice in that the preliminary washof vthe pulp and the consequent recovery of a large quantity of the residual alkali is effected in the digester itself.

The pans 42 and 43 are provided with the draw-off lines 44 and 45, flow of liquid through which is controlled by valves 44' and 45 respec-` The tank 50 is a pressure tank and liquor from the tank 50 is discharged as desired through valve 52 and line 52 to the evaporator and alkali recovery system.

The discharge side Weak liquor tank 5| is provided with the drawoff line 55 in which is interposed a control valve 55. Line 55 is connected with pump 56 and the pressure side of the pump is connected with line 51. Line 51 connects through valve 51' `to the digester feed line I0.

Wash Water, at any desired temperature, is fed to the Washing pans through the line 60 and valved branches 6| and 52.

In carrying out the process the manhole 3 is removed and chips are admitted to the digester either by gravity flow or by utilizing a chip lling device. After filling, thecover 3 is bolted on, valves 2|', 32 and 51 are closed, valves 8 and 6' are opened and pump 1 is operated to force pre-V heated alkaline liqulor from the accumulator through lines 8 and Il) to the distributor pipe I2.

As is known, the hot alkaline liquor acts relatively rapidly on the chips. To insure uniform distribution and chemical action the entering liquor is contacted With the chips substantially throughout the length of the digester by` means of the apertures I3. The liquor forced into the digester along its central axis is thus uniformly distributed throughout the body ofthe chips. The

pump 1 is maintained in operation with valves 2 I and 32' closed until the digester is filled to the desired level. Thereafter the steam circulating circuit may be operated, in a manner to be more fully described, or prior to such recirculation the pump 1 may be continued in operation until a high hydrostatic pressure of the order of 50 lbs. per sq. in. or more is established in the digester. 'Ihe preheated alkaline liquor under such' pressure rapidly and uniformly ,permeates the chips,

thus insuring equalized temperature throughout the'mass and a uniform reaction.

After the digester has been lled with liquor either under atmospheric pressure or under the high hydrostatic pressure as described,`the temperature of the mass is uniformly increased. lT o do this the valves 2|' and 32 are opened and steam. from header 30 is passed through line 3|, recirculator 24 and line 32.

. vIn these circumstances liquor is relieved from the digester through the line 2| to the separator 22. v In this element the relief separates into vaporous and liquid components. The vapor space of the separator, als noted, is connected through the line 23 to the low pressure zone of the recirculator 24. Hence the high velocity steam passing from line 3| through eductor 24 withdraws the flash steamseparated in element 22 and the mixture is then passed to the bottom of the digester. In this manner the `temperature of the liquor in the digester cycle is progressively raised to the desired reaction value.

'I'he liquor from separator 422 may be passed either continuously or intermittently through valve 25' and line 25 to accumulator 2. Line 25, it will be understood, may be provided with a4 check valve to avoid undue back pressure from the accumulator to the separator. This hot liquor thus is directly mixed with the preheated liquor maintained in the accumulator.

Because -of' the utilization of the accumulator, thus insuring adequate supply of preheated liquor, the action of the caustic liquor on the chips is very rapid. Furthermore, due to the high initial temperature of the liquor, there is very little dilution of this liquor by steam condensation. By minimizing or reducing sulch dilution, a reduction in the initial concentration of the cooking liquor is achieved while at the same time maintaining a. high yield and quality of the product, thus establishing a definite saving in soda consumption.

Furthermore, as will be appreciated, this treatment involving the initial contact of the chips with hot alkali liquor permits the employment of a two stage cook. In these circumstances after the chips have been thoroughly penetrated with the hot alkaline digestion liquor the unabs'orbed strong liquor may be drawn off to the accumulator early in the cook, in a manner more fully to be described. and may be replaced with a weaker liquor or with hot water to complete the cook and effect substantial savings in soda cost and a reduction in bleach.

After the cellulosic material within the digester I has been reacted with the hot caustic for the desired cooking period, the liquor is withdrawn in a novel manner. At the end of the cook the steam through line 30 is shut off and pump I6 is operated, valve 8' being closed. The hot liquor then is discharged through the coil I9 into the strong liquor tank. In passing through coil I9 the hot liquor gives up a considerable quantity of its heat, due to the indirect heat exchange, to the mass of strong caustic liquor within the accumulator, thus markedly preheating such liquor. The liquor withdrawn from the digester is passed thence through line-'I0 and valve I0 to the strong liquor pressure tank. The liquor entering this tank, while diminished in temperature by reason of the heat exchange in the accumulator, is still hot. From tank 50 liquor may be fed through valve 52' and line 52 directly to the evaporators. It will be appreciated that sincethe liquor to be recovered, passing through line 52, is still at a relatively high temperature, the load on the evaporators is very materially reduced, This is a particularly valuable feature of the invention, especially when the evaporator unit comprises a multiple effect vacuum apparatus.

After evaporation the liquor is calcined and causticized for the recovery of caustic soda in the well known manner, The strong caustic liquor is fed from the recovery apparatus, in the manner described, through the line 5, to the digestion system. In the accumulator, as described, it is preheated by indirect heat exchange with the hot alkaline liquor discharged from the digester and passing through coil I9.

In the present process, as has been indicated hereinbefore, the digester itself may be employed for the rst washing. To accomplish this liquor from weak liquor tank 5I is picked up by pump 56 and forced through line 51 and valve 51', line I0 and distributor I 2 to the digester. Here again the distributor functions very effectively as a Washing element inasmuch as it uniformly dis-l tributes the hot washing liquor throughout the mass of chips. The pump 56 is continued in operation until the digester is filled or substantially lled with the hot washing liquor. 'Ihereafter this liquor may be circulated through the digester tg slightly opening the steam valve in circulator It is particularly to be observed that with the structure shown in Fig. 1, the operation may be very materially expedited by pumping in the weak liquor for the first wash cycle as the hot strongv liquor is withdrawn from the bottom of the digester, that is to say at the termination of the cooking'cycle. In this type of operation, as will be appreciated, the pumps 56 and I6 may be operated simultaneously so that as the hot alkaline liquor is withdrawn from the digester through the distributor p ipe I3, the weaker wash liquor enters the top of the digester and is distributed through the upper section of the chips. In these circumstances the chips are rapidly drained, due to the actionof pump I6, and are immediately contacted with the washing fluid. After the strong caustic liquor has been removed from the digester, which may rapidly be determined by a suitable recording mechanism or by test, the pump I6 is stopped while pump 56 is continued in operation to ll the digester to the desired extent.

It will also be appreciated that if desired the first washing with the weak liquor may be carried out under any desired degree of pressure. To a'ccomplish this it is'necessary only to operate the pump 56 to the desired extent.

It will thus be seen that the several elements embodied in the digester cycle perform a number of functions or, stated differently, the digester unit is effectively utilized in a number of cycles in the operation. 'Ihus the steam recirculation line 24-32--2I serves not only to increase the temperature of the caustic liquor to bring it up to the desired reaction value, but it also functions in and adds considerably to the effectiveness of the first wash. Likewise the distributor pipe I2 serves as a feeding conduit during the cooking cycle, a drawoff line at the end of the cooking cycle, and a feeding line during the first washing cycle.

As is understood, during the first wash cycle the wash liquor takes up a considerable quantity of the residual alkali in the chips. At the termination of this rst washing cycle a portion of the now strengthened wash liquormay be discharged into the strong liquor tank through the pump I6,

line I8 and coil I9, and a digester may be blown in the usual manner to the blow-off tank 40. The pulp material from tank 40 may then be discharged to the washing pans 42 and 43 where washing is effected by hot water passed through line 60 in the well known manner. The wash liquor may be discharged from tanks 42 and 43 optionally to tanks 50 or 5I or otherwise disposed of.

As is known, although the actual cooking cycle lin a sulphate process is relatively short, a considerable period of time isconsumed in washing. Thus in most operations the net total washing time may extend from eleven to eighteen hours per charge. out the initial washingv in the digester and'particularly by initiating this washing immediately after the termination of the cook, marked economies in the washing are effected while insuring a high recovery of caustic.

The advantages of the pressure accumulator 2 in this type of operation will be appreciated. By such employment, as has been noted hereinbefore, the chips are initially contacted with caustic at a high optimum temperature, thus accelerating the cooking cycle while permitting the use of lower concentrations of cooking' liquor. The employment of the accumulator in conjunction with coil I9 as a heat exchange medium conserves the thermal values in the process.

As already mentioned, the principles of the invention may be embodied in a process utilizing indirect cooking. Such a systemis shown in Figure 2. In this structure the association of the digester, accumulator and washing system is substantially the same as that shown in Figure 1 and functions similarly. of the caustic liquor in the digester is elevated by indirect heating with steam and recirculation is effected under positive pump pressure.

In the present process by carrying In` this unit the temperature As shown, the steam passes from the header 30 through the line 80, controlled by valve 80', to the heat exchanger 8|. The condensate from the heat. exchanger passes through line 82 to an automatic selector trap 83 of any preferred type, such for example as the Martina trap. This element, as will be appreciated, functions to selectively divert pure condensate through the line 84 to the hot Well, and condensate contaminated with caustic through the line 85 either to some stage in the process or to the sewer. that where condensate is not intended to be used for the boiler feed, such trap may be omitted.

Associatedwith the exchanger 8| is a circulating line adapted to withdraw liquor from and reintroduce it to the digester. The digester is provided with a drawoff line 90, having the control valve 90', which line is connected to the suction side of pump 9|. 'I'he pressure side of the pump is connected to line 92, a portion of which is comprised in the heat exchanger 8|. Line 92 enters the digester at an upper section thereof through a suitable tight fitting 93 and terminates Within the digester in a perforated distributor section 94. While the digester recirculating line, as shown, `functions to Withdraw liquor from the bottom and reintroduce it at the top of the digester, it will be appreciated that, with suitable connections, this fiow may be reversed.

'Ihe operation of this type of `structure is essentially similar to that shown in Fig. 1 and previously described. The digester may be first lled with chips through the manhole cover 3 andv thereafter contacted with caustic liquor which has been preheated in the accumulator with hot reaction products. The digester may be filled with caustic liquorl at substantially atmospheric pressure or, as previously explained,

pump 1 may be continued in operation until a L high hydrostatic pressure is built up Within the digester. After the digester has been filled the caustic liquor is recirculated directly to and from the-digester through the circuit 90--9I- 92-8I-94. Hot caustic liquor may be thus recirculated to and from the digester until the mass is completely digested. In lieu of this type of operation, as has already been explained, a two-stage co ik may be utilized, in which vcase after a predetermined period of circulation the excess strong caustic may be drawn off from the digester and replaced with hot water or with a caustic liquor of similar or different characteristics from that first employed and at a lower concentration. v

At lthe termination ofthe cook, in the manner already described, the major quantity of the residual alkali may be washed or dissolved out of the pulp while. it is retained in the digester by forcing wash liquor through the circuit 55 56-51-|0-|3. The introduction of the wash liquor maybe effected contemporaneously with the drawoif of the cooking liquor, as has already been explained.

After the preliminary wash in the digester the pulp may be blown through open valve 4' to the blowoi tank and then subjected to. the successive wrshings in the manner hereinbefore described.

In this operation also decided economies in the recovery of caustic' are achieved by forcing strong caustic liquor, while at an elevated temperature, directly to the evaporator, thus diminishing the load on this element and accelerating the recovery. It will be appreciated that the chemical characteristics and the temperature of It will be appreciated the liquor fed from accumulator 2 to the digester may be controlled within wide limits. While the liquor within the accumulator is shown as being heated by indirect heat exchange with notreaction products, it will be understood that this type of heating may be supplemented by heat derived from separate steam coils positioned interiorly or exteriorly of the accumulator and in some circumstances, if desired, by the introduction of direct steam into the accumulator.

The length of the cooking cycle, as is well understood by those skilled in the art, will vary depending upon such factors as the initial temperature and concentration of the digestion liquor and the type of Wood. Deciduous woods may be eifectively delignifled in a considerably shorter period than coniferous woods. In the present process the cooking cycle is eifectively shortened due to the vigorous circulation and to the initial high and maintained temperature. The utilization of the preheated caustic, particularly in conjunction with a high pressure saturation further accelerates the cooking reaction.

While the invention has .been explained with relation to the employment of soda or sulphate liquors, it will be understood that the process applies equally to the several modications of such alkaline digestion liquors, such as alkaline digestion processes involving the use of adju- Vants, such as sodium sulphite, thio-sulphate and the like.

While preferred modifications of the invention have been described, it is to be understood that these are given didactically to more clearly explain and illustrate the process and not as the specific or exclusive'combination of steps which may be utilized to produce the novel results. The invention is conceived to reside in the concept of processing cellulosic materials, utilizing the advantageous factors herein described.

I claim:

l. A process of producing chemical pulp which comprises charging a mass of woody material to a digester; accumulating a large volume of hot alkaline liquor in a pressure vessel; forcing the liquor from such vessel to the digester and evenly distributing the liquor over the material in the digester; recirculating the liquor directly to and from the digester while adding heat units thereto, and maintaining such circulation until the material has been digested; withdrawing hot liquor from the digester and utilizing such liquor to preheat the liquor in the pressure vessel.

2. A process of producing pulp which comprises lling a digester with fibrous material; accumulating a large volume of hot alkaline liquor under pressure in a separate Vessel, forcing the liquor from such vessel into the digester along the central axis` thereof; recirculating the liquor directly to and from the digester while adding heat units thereto and maintainingl such circulation until the material has been digested; withdrawing hot liquor from the digester and passing it, in indirect heat exchange relationship, through the liquorl in the pressure vessel.

3. A process of producing pulp which comprises filling a digester with cellulosic material;

accumulating a large volume of hot alkaline liquor in a pressure vessel; Aforcing hot liquor from the vessel directly into the digester; passing hot liquor from the digester through'the heat exchange to preheat the liquor in said vessel, and then passing such digestion liquor to an evaporator without further cooling. y

4. In an alkaline digestion system for producliquor from the digester and passing it in indirect ing chemical pulp, that improvement which comprises utilizing hot cooking liquor withdrawn from a digester to preheat a body of alkaline liquor in a pressure accumulator, and then passing the cooking liquor while still hot to an evaporator and subsequently calcining the concentrate.

5. A process of producing pulp which comprises filling a digester with cellulosic material, establishing a large volume of hot strong liquor in a pressure accumulator; forcing liquor from the accumulator to the digester and cooking the material in the digester with such hot liquor for a predetermined period; then withdrawing the hot alkaline liquor from the digester and introducing a weaker alkaline liquor into the digester and completing the cook with such weaker liquor.

6. A process of producing pulp which comprises lling a digester with cellulosic material,

storing a large volume of hot, strong alkaline liquor in a pressure accumulator; forcing hot liquor from theaccumulator to the digesterto establish a high hydrostatic pressure on the liquor; cooking the material in such liquor for a predetermined time, then withdrawing such hot alkaline liquor and introducing a weaker liquor into the digester and completing the cook with such weaker liquor.

7. A process of producing pulp which comprises lling a digester with cellulosic material, establishing a large volume of hot, strong alkaline liquor in a pressure accumulator; forcing liquor from the accumulator to the digester and cooking the material with hot liquor for a predetermined period while recirculating the liquor directly to and from the digester; then withdrawing the hot alkalineliquor from the digester and introducing a weaker alkaline liquor into the digester and completing the cook with such weaker liquor.

8. A process of producing pulpwhich comprises lling a digester with cellulosic material, establishing a large volume of hot, strong alkaline liquor in a pressure accumulator; forcing hot alkaline liquor from the accumulator to the digester to establish a high hydrostatic pressure on the liquor; cooking the material in such liquor for a predetermined period of time -while recirculating the liquor directly to and from the digester; then withdrawing such hot alkaline liquor and introducing a weaker liquor into the digester and completing the cook with such weaker liquor.

9. A process of producing pulp which comprises filling a digester with cellulosic material; establishing a large volume ofhot, strong alkaline liquor in a pressure accumulator; forcing hot `liquor from the accumulator to the digester to establish a high hydrostatic pressure on the liquor, cooking the material in such high pressure liquor for a predetermined period of time, then withdrawing such hot alkaline liquor and introducing a weaker liquor into the digester, suchl weaker liquor being under less hydrostatic pressure than the said strong liquor, and completing the cook with such weaker liquor.

10. A process of producing pulp which comprises charging a digester with cellulosic material; establishing a large volume of hot strong caustic liquor in a pressure accumulator, forcing liquor from the accumulator to the digester in sufficient amounts to establish a high hydrostatic pressure in the digester; recirculating the high pressure alkaline liquor directly to and from the digester; subsequently withdrawing the hot heat exchange relationship withthe liquor in the accumulator.

11. A process of'producing pulp which comprises charging a digester with cellulosic material, establishing a volume of hot, strong caustic liquor in an accumulator; heating the liquor in the accumulator by indirect heat exchange with hot digestion iluids; forcing hot liquor from the accumulator to the digester, introducing steam into the .digester and utilizing such steam introauction to withdraw liquor from and recirculate 1t back into the digester.

12. A process of producing pulp which comprises filling a digester with cellulosic material;

accumulating a large volume of hot, strong liquor in a pressure vessel, forcing liquor from the ves` sel to the digester and cooking the material with the hot liquor for a predetermined period, then withdrawing thehot alkaline liquor from the digester and utilizing such withdrawn liquor to preheat the liquor in the pressure vessel; introducing a weaker alkaline liquor into the digester and completing the cook with such kweaker liquor; and during the cooking period recirculating alkaline liquor directly to and 'from the digester while adding steam directly thereto.

13. A process of producing pulp which compr'ises charging a digester with cellulosic material, establishing a large volume of hot, strong caustic liquor in an accumulator, forcing liquor from the accumulator to the digester in suiiicient amounts to establish a high hydrostatic pressure in the digester; forcing steam into the digester and utilizing the ow of steam to withdraw liquor from and reintroduce it into the digester; withdrawing hot alkaline liquor from the digester at the termination of the cooking period and utilizing such hot liquor to preheat the liquor stored 1n the pressure accumulator.

14. A process of producing pulp which comprises charging a digester with cellulosic material, establishing a large volume of hot, strong caustic liquor in a pressure accumulator; forcing liquor from the accumulator to the digester; withdrawing hot liquir from the digester -under pump pressure and reintroducing it into the digester during the cooking period; withdrawing hot digestion liquor at the termination of the cook and forcing such hot liquor in indirect heat exchange relationship through the alkaline liquor maintained in the said accumulator.

15. A process of producing pulp which comprises charging a digester with cellulosic mat'erial; accumulating a volume of hot caustic liquor in a pressure vessel, pumping the liquor from said pressure vessel to the digester and distributing such liquor into the material in the digester laterally along the longitudinal axis of the digester; subsequently withdrawing liquor from the bottom section of the digester and contemporaneously introducing a washing liquor Withdrawn liquor to preheat the alkaline liquor in the accumulator.

17. A process of producing pulp which comprises iilling a digester with fibrous material, ac-

cumulating a large volume of hot alkaline liquor under pressure in a pressure vessel, forcing the liquor from'such vessel into the digester along the central axis thereof; recirculating the liquor directly to and from the digester while adding heat units thereto and maintaining such circulation until the material has been digested; withdrawing hot liquor from the digester and passing it, in indirect heat exchange relationship, through the liquor in the pressure Vessel and additionally heating the liquor in the pressure vessel by introducing direct steam thereto.

18. A process of producing pulp which comprises filling a digester with cellulosic material, accumulating a large volume of hot alkaline liquor in a. pressure vessel; forcing hot liquor from the vessel directly to a digester; passing hot liquor from the digester, in indirect heat exchange relationship, through the liquor in the pressure vessel to preheat the liquor in said vessel, additionally heating the liquor in said vessel by lintroducing direct steam therein, and then passout further cooling.

19. A process of producing pulp .which comprises illling a digester with cellulosic material,

establishing a large volume of hot, strong alkaline liquor in a pressure accumulator; forcing hot alkaline liquor from the accumulator to the digester to establish a high hydrostatic pressure.

on the liquor; cooking the material in such liquor for a predetermined period of time while recirculating the liquor directly -to and from the digester and indirectly heating the liquor during such recirculation; then withdrawing such hot' alkaline liquor and introducing a weaker liquor into the digester and completing the cooking with such weaker liquor.

20. A process of producing pulp which comprises charging the digester with cellulosic material, establishing a large volume lof hot, strong caustic liquor in an accumulator, forcing liquorl from the accumulator to the digester in sufllcient amounts to establish a high hydrostatic pressure on the digester, recirculatin'g the high pressure liquor to and from the digester while adding heat during the recirculation; withdrawing hot alkaline liquor from the digester after a predetermined cooking period and utilizing such hot liquor to preheat the liquor stored in the pressure accumulator.

' THOMAS L. DUNBAR. 

